1. Field of the Invention
This invention relates to electrostatic chucks for retaining a semiconductor wafer during semiconductor wafer processing in a semiconductor wafer processing system and, more specifically, to connectors for connecting DC chucking voltage and radio-frequency (RF) biasing power to an electrode embedded in a chuck.
2. Description of the Background Art
Numerous electrostatic chucks are known to the art for retaining a semiconductor wafer within a process chamber of a semiconductor wafer processing system. A semiconductor wafer processing system is disclosed in U.S. Pat. No. 4,842,683 entitled MAGNETIC FIELD-ENHANCED PLASMA ETCH REACTOR, David Cheng et al. inventors, patented Jun. 27, 1989, and assigned to the same assignee as the present invention; this patent is incorporated herein by reference as if fully reproduced herein. Further a typical prior art electrostatic chuck 10 is illustrated diagrammatically and separately in FIG. 1. Chuck 10 includes a chuck body 12 of ceramic material, such as for example aluminum nitride, and further includes an electrode 14 embedded in the chuck body 12, near the top portion thereof. The embedded electrode 14 may be, for example, a molybdenum mesh electrode. The electrode 14 is coupled to a power supply through a connector 16. The connector 16 includes a first male connector member 18 and a second female connector member 20. Chuck 10 is attached to a cooling plate 22 suitably mounted to the bottom of the chuck body 12 such as for example by a suitable adhesive or by suitable bol-s not shown. The cooling plate 22 may be made, for example, of stainless steel or aluminum and is provided with a plurality of cooling channels 21 for carrying a liquid coolant for cooling the chuck 10. The first connector member 18 includes an upper solid cylindrical portion 24 extending through a bore 25 formed in the chuck body 12 and an integrally formed lower solid cylindrical portion 26 extending through a bore 27 formed in the cooling plate 22; lower cylindrical portion 26 has a smaller diameter than the upper cylindrical portion 24. The second connector member 20 is provided with inwardly extending an upper cylindrical bore 28 forming a collet 29 for receiving the lower cylindrical portion 26 of the first connector member 18 to mechanically and electrically interconnect the first and second connector members 18 and 20. The second connector member 20 is fixed within an insulator portion 11 of a pedestal base (not shown). The bottom of the second connector member 20 is connected to a source of RF biasing power 30 and a source of DC chucking voltage 32 by a connector 34 and a conductor 35.
Referring to the top portion of FIG. 1, a body of suitable electrically conductive adhesive 36 mechanically and electrically interconnects the top of first connector member 18 and the electrode 14. The first connector member 18 is made of molybdenum and is suitably plated with an electrically conductive material such as gold, silver, nickel or copper for RF current conduction, and the second connector member 20 is made of beryllium copper and may be suitably plated with an electrically conductive material such as gold, silver or nickel for RF current conduction. Upon the lower cylindrical portion 26 of the first connector member 18 being inserted in the collet 29, DC chucking voltage is applied to the embedded electrode 14 to retain a semiconductor wafer on the chuck 10 during semiconductor wafer processing and RF biasing power is applied to the embedded electrode 14 to bias the retained semiconductor wafer.
Referring still to FIG. 1, it will be understood that the above-described male and female connector members 18, 20 of the connector 16 are made separately and are assembled in a blind assembly as indicated diagrammatically by the double headed vertical arrow 37. The lower cylindrical portion 26 of the first connector member 18 is referred to as a pin, and the bore 28 provided in the second connector member 20 is referred to as a collet and their interconnection is referred to as a pin and collet interconnection. Blind assembly of the pin and collet interconnection can result in angular misalignment between the pin and collet as indicated diagrammatically by the double headed horizontal arrow 38 in FIG. 1, and such angular misalignment is illustrated in solid outline and in exaggerated form in FIG. 1A. As shown in FIG. 1A, such angular misalignment can result, in extreme cases, in point or line contact rather than surface or area contact between the pin and collet. Such misalignment can result in unwanted electrical contact resistance leading to local RF heating and unwanted reduction in DC current for DC chucking voltage with an attendant loss in semiconductor wafer retention on the chuck 10. Further, the insertion of the pin into the collet can result in unwontedly high assembly and disassembly forces which can result in the misalignment forces being transferred to the interconnection between the top of the first connector member 18 and the embedded mesh electrode 14 causing mechanical and electrical failure of such interconnection. Similarly, unwantedly high disassembly forces which may be required to remove the pin from the collet and thus disassembly forces also can result in premature failure of the electrical and mechanical connection between the top of the first connector member 18 and the mesh electrode 14 provided by the adhesive 36.
Accordingly, there is a need in the art for a chuck having an improved connector for connecting RF biasing power and DC chucking voltage to an embedded chuck electrode and which improved connector includes at least two connector members which substantially avoid the above-noted connector member misalignment problem.
As is further known in the art, certain semiconductor wafer processing requires that the chuck be operated at a relatively elevated temperature, for example, about 200.degree. C. to about 500.degree. C. Thus, and referring again to FIG. 1, upon the chuck 10 being operated at about 200.degree. C. to about 500.degree. C. and upon the first and second connector members 18 and 20 being respectively molybdenum and beryllium copper, such molybdenum and beryllium copper connector members conduct the heat to which the chuck 10 is heated, e.g., about 200.degree. C. to about 500.degree. C., with little reduction in temperature, to the bottom of the second connector member 20. Upon the bottom of the second connector member 20 being at about 200.degree. C. to about 500.degree. C., with some reduction in temperature, the electrical connector 34 and the conductor 35 applying the RF and DC biasing voltage to the connector must be able to withstand substantially 200.degree. C. to about 500.degree. C., with some reduction in temperature, which greatly and undesirably increases their cost and which connector and conductor typically are not commercially available.
Accordingly, there is a further need in the semiconductor wafer chuck art for a chuck which is operated at a relatively high temperature in the range noted above and which chuck includes a connector for applying the DC chucking voltage and the RF biasing power to the chuck electrode which includes thermal impedance which at least assists in reducing the heat transferred between the top portion of the connector and the bottom portion of the connector such that a standard or commercially available electrical connector and electrical conductor can be connected to the bottom of the connector and used to apply the DC chucking voltage and the RF biasing power to the chuck electrode.